1. Field of the Invention
The present invention relates to wafer metrology, and more particularly to generating a library of simulated-diffraction signals for use in optical metrology to determine the profile of periodic gratings.
2. Related Art
In semiconductor manufacturing, periodic gratings are typically utilized for quality assurance. For example, one typical use of such periodic gratings includes fabricating a periodic grating in proximity to the operating structure of a semiconductor chip. By determining the profile of the periodic grating, the quality of the fabrication process utilized to form the periodic grating, and by extension the semiconductor chip proximate the periodic grating, can be evaluated.
Optical metrology can be utilized to determine the profile of a periodic grating. In general optical metrology involves directing an incident beam at the periodic grating, and measuring the resulting diffraction beam. The characteristics of the measured-diffraction beam (i.e., a measured-diffraction signal) is typically compared to a library of pre-determined diffraction signals (i.e., hypothetical-diffraction signals) that are associated with known profiles. When a match is made between the measured-diffraction signal and one of the hypothetical-diffraction signals, then the profile associated with the matching hypothetical-diffraction signal is presumed to represent the profile of the periodic grating.
In general, the process of generating a hypothetical-diffraction signal involves performing a large number of complex calculations, which can be time and computationally intensive. The amount of time and computational capability and capacity needed to generate hypothetical-diffraction signals can limit the size and resolution (i.e., the number of entries and the increment between entries) of the library of hypothetical-diffraction signals that can be generated.